Hmm, well I think disqus uses javascript to load the new comments, so it's probably possible to use a greasemonkey script (or tampermonkey, since you use chrome) to load all the comments at once. Then you could just use the browser's built in search function like you were doing before. Something like
this, maybe?
It's certainly possible through the disqus API, but the coding in [insert language here] for such a thing would be beyond me, I fear.
ETA: After testing that script, it looks like it still only loads when you get to the bottom of the page--it just takes away the need to expand comment threads or click the button to load more. So helpful, but not quite as helpful as we'd like. It also needs
some patching to work right. I can help you with that if you want to do it.
OTOH, I may be making this too complicated: you can also type "site:sssscomic.com kantele" in google and it'll bring up everything on the entire site containing the word kantele (one result). Google does index disqus comments--but I don't know which pages of the site (and their associated comments) google has indexed or how fast they index it. This also works rather better as a whole-site search rather than searching specific pages. Nevertheless it probably would speed things up considerably.